Piezo-electric crystal



y 9, 1933. w. A. MARRISON 1,907,427

PIEZO ELECTRIC CRYSTAL Original Filed Dec. 19, 1928 HQHL-JL J F/QJ F/a.6

i f ,0 5 /Q it: T 5

VEN TOR W A. MARRISON A TTOPNE Y Patented May 9, 1933 V UNITED STATES PATENT OFFICE WARREN A. MARRISON, OF MAPLEWOOD, NEW JERSEY, ASSIGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK PIEZO-ELECTRIC CRYSTAL Original application filed December 19, 1928, Serial No. 327,017. Divided and this application filed November 24, 1930.

This application is a division of my copen'ding application Serial No. 327,017 filed December 19, 1928.

This invention relates to piezo-electric crystals and particularly tocrystals having a small tern erature coefficient of frequency, and method; 'of cutting such crystals and mounting them for connection in an electric circuit.

The advantages of utilizing the piezo-electric effect of substances possessing such properties have been known for some time. The uses for a constant frequency control and especially the need of such control within more rigid limits are constantly increasing. Such uses'include the control of broadcasting stations on theirassigned wave lengths, whether locally or by transmission of a wave from a central control point, and control of the frequency of local oscillations in a heterodyne receiver. Frequency control means are also useful in connection with sending and receiving sets in picture transmission and television, inorder to avoid the necessity of a synchronization channel, and similarly in systems of carrier wave telephony and telegraphy and are also important elements of laboratory reference standards.

An object of this invention is to provide a pieZo-electric resonator whose resonant frequency of vibration does not change with variations in temperature. I

A feature of this invention is a piezo-electric resonator of rectangular cross-section having a temperature coefficient of frequen cy approximately equal to zero.

Another feature of this invention is the provision of 'means for mounting such a resonator in connection with an electrical circuit so that minor adjustments in its temperature coefiicient may be made.

In the drawing,

Fig. 1 shows a plurality of piezo-electric resonators cut from a crystal, the successive resonators from left to right each being formed by cutting down the thickness of the resonator at the left, so that they are of uniform cross-sectional area;

' Fig. 2 is a diagrammatic view of a resonator adapted for predetermination of the erial No. 49?,783.

temperature coefiicient of frequency by relative adjustment of the energies of vibration in different directions with two pairs of electrodes, one pair being electrically connected through a resistance having a Variable tap, the other pair directly, and the electrodes of each pair being perpendicular to one another;

Fig. 3 is a diagrammatic view of a resoator adapted for predetermination of the temperature coefficient of frequency by variation of the spacings of certain electrodes, or a certain electrode from the resonator;

Fig. 4L is a diagrammatic View of a resonator with two pairs of electrodes, the electrodes of each pair being parallel to one another, one pair constituting an input coupling, and the other an output coupling, to the resonator;

Fig. 5 is a diagrammatic-view of a' resonator with two electrodes; and

*ig. 6 is a diagrammatic view of a piezoelectric resonator with two electrodes of smaller area than those of Fig. 5.

The stiffness, and the temperature coefii cient of stiffness, of quartz crystals, are different along different axes. The effective stiffness along any given aXis is the sum of at least two effects, one being the usual mechanical stiffness, such as exists in ordinary isotropic substances, and another being due to tl e reaction of the electric field set up withim and around a piece of mechanically strained pieZo-electrically active material.

When an elastic body is deformed in a given direction by a force applied in that direction, there is a corresponding, but smaller, deformation in the perpendicular direction, as well as a change in volume. When a quartz resonator is set in resonant vibration, there is a large periodic change of length in one direction, called the direction of vibration, and a vibration of the same fre quency in a transverse direction. The transverse vibration is due partly to the mechanical tendency of the material to maintain constant volume, partly to the mechanical coupling between the two modes of vibration, and parly to the electrical coupling between the electrodes and the resonator perpendicperature coeflicient o ular to the principal direction of vibration. Thus the eflt'ective stiffness which determines the resonant frequency of a resonator in a given mode is a complex quantity de endent 6 on the, relative dimensions along iflerent resonator axes, the orientation with respect to the original crystal axes, the size, number,

cing, and arrangement of electrodes about t e resonator, the voltage impressed upon the 10 resonator in various directions in relation to the dimensions and orientation of the resonator, and the impedance of the electrical circuitto which the resonator is coupled.

Because of the various factors above men- 15 tioned which determine the stiffness charahteristics for given modes of vibration, there results a similar complexity as to the temperature coefiicient of stiffness for the corresponding modes of vibration, hence it tends to. result that the temperature coeflicient of stiffness, and, therefore the frequency, of a resonator, in agiven mode, may be varied over a considerable range by suitably proportioning the resonator.

' Theinherent temperature coeflicient of frequency: is difierent alon an electrical axis of a crystal from that in a perpendicular directionalong a crystallo aphic axis.

plates of relatively arge area cut so a that their long dimensions are parallel to the optical and electrical axes, have a positive temperature coeflieient of frequency for vibration 0.10 the shortdimension, while thin plates of re atively large area cut so that 35 their long. dimensions are parallel to the optical andcrystallogrephic axes have a I negtive coefiicient. If a plate is cut from a quartz crystal in the plane of the optical and electrical axes, as above, but having: sizflicient thickness in proportion to its cross sectional area, such as that shown on the left in Fig. 1, it will be found to have a small negative temperature coeflicent of frequency. If a portion of its surface is removed, the crosssectional area remaining constant while the thickness diminishes, the temperature coefiicient becomes less negative, or more nearly positive. If successive tests are made with progressively decreasing thicknesses of resonators having the same cross-sectional area, such as those shown from left to right in Fig. 1,. it'will be found that as the proportion of the thickness of the resonators to the cross-sectional area decreases, the temperature coefiicient. of frequency of the resonator for vibrations along the short dimensions will rise from negative through zero :to positive values. This is apparently due: to the mutual dependence of different vibrations and to the opposite temperature coeflicients' in perpendicular directions. 1 Kit is desired to cut a quartz resonator of rectangular cross-section designed to vibrate at a desired frequencfy and having a zero temfrequency, the first step is to cut such a plate from a crystal arallel to the optical and electrical axes of t e crystal, of slightly greater thickness than re quired for this frequency. The plate is then ground to the proper thicknessto vibrate at a slightly lower frequenc than the frequency desired. The sides of t e crystal are then u cut downto decrease the cross-sectional a tests being made at suitable intervals no the point is reached where the plate has a slightly negative temperature coeficient of frequenc A final adjustment of tem ra ture coe cient of frequency ,is then ma e by grinding to the proper thickness. The tests mentioned are of course all at the same temperature'so that the cry'staLwill vibrate at a desired frequency at a desired temperature, It is necessary to make the adjustments in three steps-instead oftwo because the first adjustment of frequency has an effect on the temperature coeflicient, and the adjustment of the temperature coeflicient has a. very slight effect on the frequency. If the exact 7 dimensions are known for a desired fre quency with a zerogte coeficient at a given orientation of the resonator with respect to its crystal axes, it may be cut .directly to these dimensionsin two steps. In Figs. 2 to 6 inclusive there are shown various methods of associating a 'l'eeoltlt fllfv with electrodes and of connecting the elec= trodes toan electrical circuit, I It has been previously. noted herein that a piezo-electriri:fi resonattfn-freq tends to lave tem perature coe cients o uenc o signs in the cases of a resonatzr cut in the plane of the optical and electrical axesianda resonator cut in the plane of the optical and crystallographic axes. The resultant'temiperaturecoeflicient of the resonator depends upon the extent of itsvibration and thecorre-g spending temperature coeflicients in both rections. Analogously tothe above, the 0pposite sign rule is ap licable to the case of vibrations parallel to th the electrical and a. crystallographic axes "a given resonator. It is therefore possible to adjust the ooeficient of a resonator by. exciting it to greater vibration in one direction than in the other; This ma be done to a certain extent by means 0 a differential adjustment of the. voltage applied to the perpendicular surfaces as shown in Fig. 2, or exam 1e, where 1 a resonator Q has four 'electr es E, as shown, and two adjacent electrodes are com nected through a potentiometer P to which the exciting voltage is applied. 7 By moving the contact point alon the potentiometer in a desired direction, t e tern rature ieiitof frequency may be a justed accoardi 1n 5 4 The relative amount of vibrationin an direction, and hence the temperature cient of frequency of the resonator, may also be controlled to a slight extent by connect" ing the electrodes directly instead of through the potentiometer, and adjusting the relative potentials applied by changing the spacing between the electrodes and the resonator or by changing the area of the crystal exposed to the electrodes. The coupling to the crystal may be Varied and its temperature coefficient of frequency adjusted to a slight extent by changing the spacing of the electrodes or by the other means suggested above by means of the coupling arrangements shown in Figs. 3 and 4.

The size of the electrodes has also been mentioned as affecting the temperature coefficient of frequency of a piezo-electric resonator. It is possible to change the coefficient of the resonator, or, if it has a low coefficient. it is possible to adjust it to zero, by changing the size of the electrodes. This is shown in the organizations illustrated by Figs. 5 and 6 in which the resonators will have different coefficients due to the difference in size of the respective pairs of electrodes.

Advantages of resonators which have a zero temperature coefiicient of frequency are that the necessity for temperature controlling means is avoided, and furthermore as the resonator heats up due to load applied to it, the frequency does not change due to either the initial heating or to variations in load.

The various methods of adjustment of the temperature coefficient mentioned herein in connection with Figs. 2 to 6, are partlcularly useful in the final adjustment of a resonator. It may be that in preparing a resonator to have a zero temperature coefficient in the i methods than by further shaping.

What is claimed is:

1. The method of reducing the temperature coefficient of frequency of a piezo-electric resonator having two pairs of electrodes associated therewith for exciting different modes of vibration of said resonator which comprises controlling the relative vibration in the diffirent modes of said resonator by varying the relative potentials applied to said pairs of electrodes.

2. A piezo-electric resonator plate of rectangular cross-section cut from a quartz crystal, the plane of said plate being parallel to the optical and electrical axes of the crystal and its dimensions so related that the resonator has substantially a zero temperature coefficient of frequency.

3. A quartz crystal piezo-electric resonator plate of rectangular cross-section, the dimensions of which are so related that it has a very small temperature coefficient of freparallel to the optical and electrical axes of I 0 the quartz crystal from which it was cut, the dimensions of this resonator being so related that it has very small temperature coefficient of frequency, two pairs of electrodes to excite and control the vibration of the resonator in two different directions, means for applying electric potentials to these electrodes in such a manner that the temperature coefficient of frequency of the vibrations of the resonator is substantially zero at a specified temperature.

5. A quartz crystal piezo-electric resonator plate of rectangular cross-section the principal plane of which is parallel to the optical and one of the electrical axes of the quartz crystal, the areea of said resonator plate being so related to its thickness that the resonator has a substantially zero temperature coefficient of frequency.

6. A rectangular shaped piezo-electric resonator plate cut from a quartz crystal in the plane of the optical and one of the electrical axes of the crystal, the length, width, and thickness of said resonator plate being so related to each other that the temperature coefficient of frequency is substantially zero.

7. A quartz crystal piezo-electric resonator plate of rectangular cross-section the principal plane of which is parallel to the optical and one of the electrical axes of the quartz crystal and having its dimensions so related that it has a small temperature coefficient, two pairs of electrodes therefor for exciting different modes of vibration of said resonator and a potentiometer connected to said two pair of electrodes for controlling the relative potentials applied to the two pairs of electrodes.

In witnesss whereof, I heretunto subscribe my name this 20th day of November, 1930.

WARREN A. MARRISON. 

